If I hear your question correctly, it's the concept that any cross-section produced by intersecting a plane (geometric plane, not airplane

) at right angles to the aircraft's longitudinal axis have the same total area as any other section.
A great comparison can be made with the F-102 and its 'Y' version prototype. The prototype did not have the area rule applied, the production aircraft did.
Here's a YF-102, (image)
and here's an F-102. (image)
Notice how the fuselage of the production aircraft slims at the point where the wing is thickest. Total area of the cross-section is the same at any point. This reduces drag at supersonic speeds. For some reason.